View Full Version : Role of Carbon Dioxide in Mead (wine)

01-17-2014, 11:02 PM
I found it at http://www.winewisdom.com/articles/techie/role-of-carbon-dioxide-in-still-wines/.

Hopefully it is acceptable forum protocol to cut/paste this type of information.

I am sharing the article to start a discussion about role of C02 post fermentation in Mead so that I can discover if degassing after primary is a good thing or no. I am familiar with the mead vs wine vs beer debate.
Would we consider a big dark fruit melomel a "red" wine?

For better and worse, carbon dioxide (CO2) is implicated in several aspects of still wine making and flavour profile. Apart from imparting a spritz to a still wine, are high levels of CO2 implicated in other issues of reductive winemaking? How does one separate the good from the bad, leaving the ugly purely as a matter of personal opinion?

Carbon dioxide (CO2) is produced by yeast as a natural product of fermentation. It’s great for keeping oxygen away from must and wine.

Received wisdom has it that CO2 becomes perceptible to the human palate at around 1g/l as a slight spritz on the tongue, akin to the sensation given by Muscadet sur lie. At around 2g/l wine is legally semi-sparkling (see below).

Its moderate presence helps to freshen a wine and gives a perception of slightly elevated acidity. “higher CO2 gives a crisper wine, with lower dissolved oxygen, less flavour intensity, and an addition to texture.” a bit of CO2 helps to preserve the wine a little, so you can lower sulphur dioxide (SO2) levels fractionally.

At too-elevated levels it makes the wine spritzy and can lead to reductive notes especially in reds. CO2 management activist, Abrie Bruwer, of Springfield Estate in South Africa, says “CO2 on the palate belittles a still wine. It makes the wine thinner and masks the nose. Think how difficult it is to assess the nose of sparkling wines.”

Julian Grubb, head winemaker of Chilean VIA Wines has winemaking objectives of “1000 – 1100 mg/l CO2 in aromatic whites, 800mg/l in and less than 500mg/l in reds. CO2 can make tannins sharper, more aggressive.

“Levels should be appropriate for the style of wine or organoleptically not apparent.

Reductive winemaking – winemaking which actively avoids oxygen – generally results in higher levels of CO2 in the wine, from retained fermentation-derived CO2 and from external sources of CO2 which have dissolved into the wine.

During fermentation, naturally produced CO2 protects the fermenting must from oxidation. After fermentation, a deliberate management policy must be adopted, depending on wine style.

Carefully controlled use of CO2 in the winery can optimise levels of oxygenation in the wine making process. Blanketed over the surface of wine, CO2 can help prevent oxidation and the growth of spoilage organisms. But it dissolves more readily in wine at the lower temperatures usually used by winemakers who work reductively. This makes it a potential problem for red wines, which need much less dissolved CO2. Additionally, as wine warms up, CO2 comes out of solution and pressure may build if there is no escape route.

“We use CO2 in bottles to work more reductively. We add it in the tanks, in trucks, during filtration etc., to remove oxygen. We also use a lot of dry ice. Its temperature is very low, so it is good for both cooling down grapes and keeping oxygen away;

Reh Kendermann are big exporters to the UK. Hofmann says: “we are looking for around 1 g/l CO2 in the wine for the bottling of whites and less than 500 mg/l for reds. Oxygen in the must destroys the pyrazines. Ascorbic acid scavenges very quickly, binding with SO2.

“We test CO2 before bottling and will blow off some if it is above 1400mg/l.

Harrop emphasises a style-specific approach to CO2. “Although some grape varieties e.g. riesling, sauvignon blanc, appear to support relatively high CO2 levels, a CO2 strategy should be predicated on wine style and vintage characteristics.”

On reds, Bruwer uses CO2 during the cold soak – it means volatile acidity takes a little longer to form. But during fermentation, Bruwer’s policy is to remove CO2: “we pump over with a fan blowing onto the cascade to give O2. Yeast under hot conditions produces glycerol if O2 is present. We want glycerol, rather than just alcohol, so we give O2.”

Taylor confirms that noticeable CO2 in still red wine is most likely indicative of a problem.

Noticeable CO2 could indicate a secondary fermentation, especially with the apparent trend to bottle some high volume reds with sufficient residual sugar to stimulate a refermentation.

Taylor says: “an increasing area is that of bottling red wine before it’s finished the malolactic fermentation. This can cause big problems. Not only do you get a slight spritz, but the bacterial damage can be significant. A heavy deposit is formed which might remain in suspension giving a cloudy wine. As the bacteria utilise the available SO2, oxidation occurs. There is much individual bottle variation. This is occurring right across the quality spectrum. Likely origins include the pressure to get wines to market earlier, or logistical pressure on tank space.”

Taylor sees this phenomenon on some very nice wines. The bacteria can remain viable in bottle for some time, 12 to 18 months; it takes maybe a combination of temperature and a drop in SO2 to a point where the bacteria can work.

Bailey warned of the potential impact of high CO2 levels for bag-in-box. “Above 600-800mg/l and the bag may swell when the temperature rises as CO2 comes out of solution. From the outside, it looks like a refermentation.”

Regarding screwcaps, Harrop suggests virtually no CO2 escapes, so a wine bottled at 1.2g/l stays that way, so with a noticeable spritz, but “with 1.2 or 1.1g/l at bottling under cork, after shipment a wine generally loses 100-150mg over 4-6 weeks, depending on the quality of closure, diameter of bottle, temperature etc., so there’s no over-spritz.”

The significance of the temperature of storage and logistics has been reported elsewhere – as wine heats, for example when containers move over the equator, CO2 comes out of solution and risks pushing out driven closures.

Active use of CO2 is both the friend and the enemy of reductive winemaking and needs to be critically monitored throughout the winemaking process to ensure optimum wine health.

When must and wine is treated reductively, it becomes especially vulnerable to oxidation, so every process after the first reductive process must be done reductively. CO2 use is an integral part of oxygen avoidance, as is cold temperature storage where CO2 retention is higher and risks include reductivity in the wine. If left unchecked, the risk is for reductivity to progress to irreversible mercaptan form.

With the trends to make more reductive styles, to minimise the time from harvest to market and to drink the youngest wines available, the use of potentially more reductive closure types has implications for the final wine analyses immediately prior to bottling. If “too” reductive a closure is used any present reductivity may persist and develop further. A bit more time after bottling or a “less” reductive closure may allow more opportunity for gases to equilibrate inside and outside the bottle.

A CO2 policy for still winemaking and bottling becomes increasingly relevant.

So my question is ................. for MEAD making is C02 relevant or is degassing the way to go?



01-18-2014, 08:09 AM
See, I don't follow the point of the question that you're trying to ask Steph ?

While there will be, in some cases, CO2 left dissolved in the wine i.e. for sparkling wines or in some others, that it's not "degassed" away for other non-carbonation reasons, it can't be ignored.

The stuff in the linked site, while of interest, is "old news" to many.

Initially, you can't escape CO2, given that it's one of the by products of fermentation. Generally, oxygen (or air) is only incorporated in the early stages, to help with yeast/colony development. The guidance to aerate down to the 1/3rd sugar break is rather arbitrary and I don't know whether any proper, in depth, research has been done to define the 1/3rd break as the ideal point to move the wine into the anaerobic stage of making i.e. to encourage the yeast to only make alcohol and CO2, rather than developing the colony to a larger size etc.

Many people, wrongly, describe early stage aeration, as "de-gassing". The primary reason for stirring/agitation of a must is just that, for aeration. A side effect of that is to nucleate out some of the dissolved CO2, which in turn comes out as bubbles/gas and has the additional (potential) benefit of helping to prevent excessive foaming/eruptions from a fermenter.

Whereas it's just as easy to view the need for that practice (de-gassing, rather than aeration of must) as poor practice in the making i.e. the use of an appropriate type/size of fermenter negates the need for de-gassing purposes. If you have the right kit, you can oxidate a must without stirring/agitating, just bubble pure oxygen through an airstone device (difficulty obtaining a source of compressed oxygen can prevent this method, leaving stirring/agitation as the only option at this stage). Equally, a lidded bucket can reduce the likelihood of an eruption as it allows for a greater air/liquid interface, so that any fruit or fruit debris that might form a cap, can break up a bit allowing any trapped gas out, as long as you haven't filled it too close to the top, as some foaming expansion is possibly/likely. At the same time, a lot of us will have seen examples of wines or meads being made in 1 gallon glass type fermenters, either by friends or relatives. Yes they can be used, but it doesn't matter how "pretty" they might seem, there are practical reasons not to use them, well at least initially.

Then, once the ferment has completed, it depends on whether you will need to use extra fruit to get the flavour profile you're aiming for, as to whether you might need to apply some de-gassing technique. Obviously there will be some loss of dissolved CO2 when racking and if there's any tolerance to produce further alcohol left within the yeast, then sulphites and sorbate would be needed to prevent further fermentation of any fruit sugars added for flavouring reasons. Plus there would/could be some further natural "off gassing" while the fruit is in the batch, which is good and provides a blanketing layer for prevention of oxidative damage (some people worry enough to apply a blanket of CO2 from an external source - IME, they've often made beers first and can be a little obsessed with protection - though with beers, that kind of process is often necessary).

As for whether it's necessary in connection of the flavour of the brew, a beer analogy is handy. With the lager type beers, that tangy sort of flavour alluded to in the linked article is often part of the flavour profile of the brew, whereas in many other styles of beer, they're basically served flat/still, and the only reason that you know there is some dissolved CO2 is because of head formation when they're poured/served, but most of it come out of the brew quite quickly and doesn't really make up part of the flavour profile.

It's hard to say, whether many wines aren't actively degassed, for shipping or protective reasons i.e. if you have no control over the shipping container (and by that I mean the bottles and their environment as they're moved), then a little dissolved CO2 that might come out of solution can be helpful as there's often no telling how long it will take to get the brew from the final processing containers (fermenters, storage tanks, etc) to the glass on the consumers table, where the consumer will expect it to be at it's peak of flavour.

A chap I know of, who does know a lot about wines, feels he can detect the presence of unremoved CO2 in wines sold as "still". He will often taste a newly opened bottle, then gets his "vacuvin" and a stopper, and will happily show others how much CO2 will come out of that bottle, offering a taste of the wine as "opened", and then actually pouring the guest a glass of the wine after he's "pulled" the gas out.

The difference in taste can sometimes be quite dramatic. I've found I often prefer it, so I routinely de-gas my meads.

Obviously, with sparkling wines and meads, the dissolved CO2 is there for an entirely different reason, but if you taste the difference between a champagne or other decent sparkling wine, and the same one but after it's been allowed to got flat, or had the CO2 deliberately removed, you would note that they're often quite bland tasting and that the CO2 is there for flavour as well as bubbles.........

Hence there's no "way to go" per se, it's up to you how you find that you enjoy your brews unless you're making something for a specific reason/aim......

Oh, and as for calling something that's been made with red fruit/juice a "red", that's likely up to you. After all, what is the difference between a red and a white ? mainly that the red is made using slightly different technique to preserve the pigmentation and some of the different flavours that come from that different technique. Many whites originated in more northerly areas and with cooler ferments, whereas the reds came from further south with warmer ferments (yes, that's pretty much a wide generalisation, especially considering commercial production techniques and capabilities these days).

So there'd be no real reason not to call something from red/black fruit a "red" mead if it's come out that colour but some normally "red" looking fruit don't actually have much pigmentation and it's quite hard to get colour from the fruit alone, strawberry being a good example, red raspberry being not such a good one, but it's easy to lose the pigmentation from red raspberry like with strawberry.........

01-18-2014, 11:47 AM
Thank you Fatbloke. I very much enjoy reading your analysis.

The whole thread for me started when I was trying to learn about how to prevent oxidation. I read using C02 was beneficial to displace oxygen in a carboy during racking. In other articles I saw references about the importance of degassing to remove C02 before bottling. And of course, C02 is bad during primary fermentation. It all seemed very contradictory.

I think what I have learned is

C02 has an important role to play in making still meads "depending" on timing

Primary fermentation - C02 is undesirable and degassing / oxygenation of the must until the 1/3 sugar break is highly recommended.

Post primary but before bulk aging and bottling - C02 is a tool that is used to reduce the possibility of oxidation. Blanketing the carboy with C02 during racking is one way to achieve this.

During bulk aging- Remaining C02 should be eliminated (unless you are making a sparkling mead,) the final amount dependent on the style of mead. Too much C02 can cause a number of issues so reducing it using degassing is necessary. Depending the style of wine (or mead) may play a role in the amount of C02 left remaining. The gooseberry/lemongrass mead I am just starting might benefit from just a hint of C02. The big fat dark cherry one I have in bulk aging probably not so much.

Removing C02 (degassing) post fermentation should be done in a manner so as not to introduce oxygen. Some type of vacuum pump is needed. This could be as simple as a hand held manual pump like is used to remove air from a bottle of wine after opening (but might take a while on a 6 gallon carboy,) an inexpensive $15 FreshSaver (don't use your expensive FoodSaver counter model) or a $200 vacuum pump. Degassing occurs naturally for free over a long term bulk aging period.

During bottling -- The final method of closure (cork, screwtop, cap) may also play a role in letting C02 escape.

Am I on the right track ?


01-18-2014, 03:34 PM
Removing C02 (degassing) post fermentation should be done in a manner so as not to introduce oxygen. Some type of vacuum pump is needed. This could be as simple as a hand held manual pump like is used to remove air from a bottle of wine after opening (but might take a while on a 6 gallon carboy,) an inexpensive $15 FreshSaver (don't use your expensive FoodSaver counter model) or a $200 vacuum pump. Degassing occurs naturally for free over a long term bulk aging period.

I'm liking my new 3 gallon glass carboy, I can sit it on a counter top and spin it a few times and al the C02 comes up to the top.

I don't think I'll try that with my 6 gallon carboy though.

Medsen Fey
01-22-2014, 08:43 PM
I think the issue of how much CO2 dissolved in your final product and bottled under your closure is a very good question. There may be some meads that do benefit from having a touch of CO2. It is easy to test by using a Cornelius Keg. You can pressurize with just enough CO2 to allow it to be pumped out, or you can increase the amount to make it pettilant.

In my experience, I don't like having CO2 in my meads. I like them fully degassed, and I like to pump them out of a keg with nitrogen or argon. You may find with some recipes, you like a little. Once you find an amount you like, you can put that amount in a keg, and bottle it cold to keep it in solution. I'll be quite interested to hear what you discover along the way should you decide to test this further.

As for one of your other points, CO2 during fermentation is irrelevant. Unless it is at greater than 1 atm of pressure, it does not inhibit yeast function. Under an airlock, it simply escapes as it is produced and the solution can hold no more. If you have it in a pressure vessel, increasing the CO2 pressure will slow yeast down.

01-22-2014, 10:06 PM
Great points Medsen Fey. Thank you for the point about C02 during fermentation. I will update my SOP.

Regarding testing the theory of C02 in styles of mead I think what I will do is bottle the dark cherry absolutely still after thorough degassing.

The gooseberry/lemongrass melomel with alfalfa honey I am making is being styled as a mock Sauvignon Blanc - one of my favorite wine varietals. Although it is a still wine - you will typically find upon opening a bottle that it has just a tiny bit of carbonation which enhances the flavor and aroma. Many individuals describe the taste of Sauvignon Blanc as gooseberry like, lemony, grassy. While I could have bought a Sauvignon Blanc wine kit and just made wine I thought it would be more fun to emulate it making a melomel. We will see how close I come.

Regarding closure method- I plan to use Zorks. Prior to purchasing them I did a search on GOTMEAD and found a study someone did on the difference between corks, caps and zorks. One of the comments of the study is when opening a bottle corked using a ZORK there is a quick "hiss" from the escaping carbonation --- the test used a still mead I believe. Maybe I will cap a bottle and also use a regular cork in a bottle and see


Medsen Fey
01-22-2014, 10:30 PM
If you really want to preserve a specific level of carbonation, a crown cap is probably the most reliable way to do it.

01-23-2014, 09:10 AM
And as for replacement caps, I'm looking to try some "Nova Twists" caps, then I wouldn't have to worry much about screw top bottles..........